1. Field of the Invention
The present invention generally relates to metal melting furnaces and, more particularly, toward a method and system for controlling a metal melting furnace based upon sensed molten metal heights and temperatures to permit more efficient operation of the furnace.
2. Description of Related Art
Gas fired metal melting furnaces are well known in the art. One such type of furnace, illustrated in FIG. 1, includes a charging tower 10, a melting area 12, a molten metal holding area 14, and a launder 16 that leads to a ladle or dispensing area 18.
The charging tower 10 is vertically above the melting area 12, and includes a removable cover such that new charging material (i.e., ingots and scrap) to be melted can be introduced into the charging tower 10. Since the charging material is disposed in the tower 10 and vertically above the melting area 12, the charging material is pre-heated prior to reaching the melting area 12, and is thereby brought to the melting temperature more quickly and efficiently.
The melting area 12 disposed at the bottom of the charging tower 10 includes a series of melting burners 20 (only one shown) that heat and melt the charging material. The melting burners 20 are operated at either a high or a low setting. The high setting is matched to the furnace capacity, and essentially corresponds to full-on operation of the melting burners 20 and the furnace. The low setting is a holding setting. Thus, the melting burners 20 are operated either full on, to melt metal as quickly as possible, or at a low setting. It will be appreciated that the furnace is most efficiently operated when the rated capacity is equal to the load, in which case the melting burners are continuously operated at their high or full-on setting. It will further be appreciated that due to varying metal demands, operation at the rated capacity is discontinuous or intermittent. Operation of the melting burners 20 in either the high or low setting is controlled based upon sensed level of the molten metal in the metal holding area 14 or the launder 16, as described hereinafter.
Melted metal flows from the melting area 12 at the bottom of the charging tower 10 into the molten metal holding area 14. The molten metal holding area 14 defines an enclosure including a refractory brick-lined bath that holds a volume of molten metal 22. A holding burner 24 is disposed above the molten metal bath, and is controlled so as to maintain the temperature of the molten metal in the molten metal bath at a predetermined temperature, typically around 1380° F. The holding burner 24 output is adjustable, within limits, based upon sensed metal temperature so as to maintain the molten metal temperature within predetermined desired limits.
Molten metal level is sensed by a series of probes 26. The probes 26 have different lengths so as to project downwardly different amounts. As shown in FIG. 3, the probes typically include a high-high level probe (HH), a high level probe (H), a low level probe (L), a low-low level probe (LL), and a ground probe (G). Such an arrangement of probes 26 is well know to those skilled in the art.
The high-high level probe (HH) senses a condition in which the level of metal in the bath is too high, and in which the melting burners 20 are turned off (i.e., e-stop the melting furnace) to prevent overfilling the molten metal bath. Essentially, the high-high level probe (HH) is a failsafe probe to help prevent overflowing the bath with molten metal.
The high level probe (H) senses a high-normal level of molten metal in the bath. The high level probe (H) is thus a control probe, and signals from the high level probe (H) are used by the controller 28 to control operation of the melting burners 20, i.e., to turn the melting burners 20 to the low setting or condition.
The low level probe (L) senses a low level of metal in the bath. The low level probe (L) is a control probe, and signals from the low level probe (L) are used by the controller 28 to control operation of the melting burners 20, i.e. to turn the melting burners to the high setting or condition. The low metal signal from the low level probe (L) also indicates an initial low metal level, which the controller uses to generate warning signal for the operator.
The low-low level probe (LL) is the corollary of the high-high level probe (HH), and indicates that the level of metal in the bath is too low, evidencing a problem in the charging tower 10 that requires attention from the operator. Such a problem could be a blockage of charging material, preventing the charging material from reaching the melting burners 20, or simply that the charging tower 10 is empty.
The ground probe (G) is typically identical in length to the low-low level probe, and provides a reference point against which other length measurements are judged. More specifically, electrical current is supplied to the probes and, when the ground probe (G) and a particular level probe (L-HH) are touching the molten metal, completes a circuit that generates a signal identifying the level of metal in the bath.
Thus, the level of metal ordinarily fluctuates a distance (X) between the low level (sensed by the low level probe (L)) in which the melting burners 20 are operated full-on, and the high level (sensed by the high level probe (H)) in which the melting burners 20 are operated on low. Unfortunately, repeated cycling or rising/falling of the metal level in the bath between the high level and the low level erodes the refractory lining (RL) of the metal holding area 14 and, over time, requires rebuilding of the metal holding area lining. Such erosion (E) is schematically illustrated in FIG. 1B. Naturally, rebuilding the refractory lining (RL) is expensive and requires the metal melting furnace to be taken off-line for an extended period of time. Accordingly, it is desirable to reduce erosion of the refractory lining (RL) and thereby extend the time between rebuilding of the refractory lining.
With respect to FIG. 2, the melting furnace includes a control system including the controller 28, an air blower 30, a series of air valves 32, 34 (only two shown), a series of diaphragm-type gas valves 36a, 36b (only two shown), a temperature sensor (thermocouple; TC) and the probe-type level sensors 26, described previously. The air valves 32, 34 are butterfly type valves including a motor that is operable to drive the butterfly valves open and closed. The melting burner air valve 34 is a two position valve that is moved into a either a full-open position, corresponding to the high or full-on setting of the burner, or a closed position (slightly open) corresponding to the low setting of the burner. The holding burner air valve 32 is a proportioning valve that may be moved to a position between the full open and closed (low) positions. Each of the air valves 32, 34 is associated with one of the gas valves 36a, 36b and with one of the melting or holding burners 20, 24.
Thus, the holding burner 24 has its own dedicated holding burner air valve/holding burner gas valve combination. Similarly, each melting burner 20 has its own dedicated melting burner air valve/melting burner gas valve combination. The controller 28 receives signals from the temperature sensor (TC) for controlling operation of the holding burner air valve/gas valve, and from a charging tower atmosphere temperature sensor (not shown) with interlocks from the level sensing probes 26 for controlling operation of the melting burner air valves/gas valves, and for operating various alarms and indicators in response to high-high and low-low molten metal conditions, described briefly hereinbefore.
The air blower 30 is continuously operated, and supplies air to each of the air valves 32, 34. The melting burner air valves 34 are two positions valves, and are either in a low open or a high open setting, corresponding to low and high operation of the melting burner 20, respectively, as noted previously. In either case, an air stream flows from the melting burner air valves 34 to the associated melting burner 20. Part of the air stream flowing to each melting burner 20 is tapped off to the associated melting burner gas valve 36a, which is a diaphragm valve, and serves as a pneumatic or air signal that controls opening of the diaphragm valve and communication of gas to the melting burner 20.
As is known in the art, the diaphragm valve 36a opens an amount that corresponds to the air signal provided to it. Thus, when a low amount of air is provided (corresponding to the air valve 34 being at the low setting), the diaphragm valve 36a opens a first amount so as to supply a first, low amount of gas to the melting burner 20. The first, low amount of gas is tuned to the air flow (i.e., air flowing to the melting burner 20 from the melting burner air valve 34) and, thus, the correct air/gas ratio is supplied to the melting burner 20, and the melting burner 20 is efficiently operated in the low setting. Similarly, when a high amount of air is provided to the diaphragm valve 36a (corresponding to the air valve 34 being at the high setting), the diaphragm valve 36a opens a second amount so as to supply a second, high amount of gas to the melting burner 20. The second, high amount of gas is tuned to the air flow (i.e., air flowing from the air valve 34 to the melting burner 20) and, thus, the correct air/gas ratio is supplied to the melting burner 20, and the melting burner 20 is operated in the high setting.
The holding burner air valve/gas valve may be operated identically to the melting burner air valve/gas valve, described above. However, and as noted previously, it is also known in the art to use a fully proportioning butterfly valve for the air valve 32, and to adjust the opening/closing amount of the air valve 32 via a motor that is actuated by the controller in response to sensed metal temperature. Such a known air valve actuating mechanism includes a link rod assembly (not shown) that is secured to the air valve 32 and mechanically establishes the high and low open positions for the air valve 32. In any event, the air stream flows from the holding burner air valve 32 to the holding burner 24. Part of the air stream flowing to the holding burner 34 is tapped off to the holding burner gas valve 36b, which is a diaphragm valve, and serves as an air signal for the diaphragm valve 36b. 
The holding burner diaphragm gas valve 36b receives the air signal tapped off from the output of the holding burner air valve 32 and opens a corresponding amount, and the correct air/gas mixture is supplied to the holding burner 24. Unfortunately, adjusting the link-rod type assembly is a manual operation that is imprecise and problematic. Notably, manually adjusting one of the high or low setpoint changes the other setpoint (i.e., low or high) and requires repeated readjustment and re-checking with each change. Improper adjustment of the link rod assembly causes the high and low setpoints or limits to deviate from the actual high and low opening positions of the air valve 32, and may result in undue cycling of the metal temperature between the high and low limits. Such cycling is believed to cause undue oxidation of the melted metal, and is therefore to be avoided. Therefore, there exists a need in the art for an improved and simpler control over the holding burner output, and resulting better control over the temperature of the molten metal.